Field
Exemplary embodiments of the disclosure relate to a light emitting diode (LED) drive circuit with improved flicker performance and an LED lighting device including the same. More particularly, exemplary embodiments of the present disclosure relate to a light emitting diode (LED) drive circuit with improved flicker performance, which can reduce deviation in light output during operation intervals of a sequential driving type alternating current (AC) LED lighting device by removing non-luminous intervals of LEDs, and an LED lighting device including the same.
Discussion of the Background
LEDs are generally driven by direct current (DC). DC driving requires an AC-DC converter such as an SMPS and the like, and such a power converter causes various problems such as increase in manufacturing costs of lighting devices, difficulty in size reduction of the lighting devices, deterioration in energy efficiency of the lighting devices, and reduction in lifespan of the lighting devices due to short lifespan of such power converters.
In order to resolve such problems of DC driving, AC driving of LEDs has been suggested. However, an AC driving circuit causes not only a problem of reduction in power factor due to mismatch between input voltage and output power of the LEDs, but also severe flickering perceived by a user in the case where non-luminous intervals of the LEDs are extended.
FIG. 1 is a conceptual view illustrating a flicker index. A definition and regulation of the flicker index as a reference flicker level in accordance with the Energy Star specifications will be described hereinafter.
(1) Definition of Flicker
Flicker means a phenomenon that brightness of lighting is changed for a certain period of time, and severe flicker can be perceived as shaking or flickering light by a user. Flicker is generally generated due to a difference between a maximum light output and a minimum light output for a certain period of time.
(2) Types of Flicker Index
a) Flicker Index: As shown in FIG. 1, the flicker index means a value obtained by dividing an area (Area1) above the level of average light output by the total light output area (Area1+Area2) on a light output waveform of one cycle. Thus, the flicker index is a value numerically indicating frequency of illumination above the level of average light output in one cycle and a lower flicker index indicates a better flicker level.
b) Percent Flicker or Modulation Depth: Percent flicker refers to a value numerically indicating a minimum intensity of light and a maximum intensity of light for a certain period of time. Such a percent flicker can be calculated by 100*(maximum intensity of light−minimum intensity of light)/(maximum intensity of light+minimum intensity of light).
(3) Flicker Level in Accordance with Energy Star specifications                Light output waveform ≥120 Hz        Flicker index ≤frequency×0.001 (at Max. Dimmer, excluding flicker index at 800 Hz or more) (thus, flicker index at 120 Hz≤0.12)        
(4) Study Result on Percent Flicker
Study reports regarding the percent flicker say that
Percent flicker <0.033×2× frequency or less indicates no-influence intervals, and
Percent flicker <0.033×2× frequency or less indicates low danger intervals.
As described above, the issue of flicker level is of increasing concern in performance of LED lighting devices.
FIG. 2 is a block diagram of a conventional four-stage sequential driving type LED lighting device and FIG. 3 is a waveform diagram depicting relationship between drive voltage and LED drive current of the conventional four-stage sequential driving type LED lighting device shown in FIG. 2. Next, problems of the conventional LED lighting device will be described with reference to FIG. 2 and FIG. 3.
First, as shown in FIG. 2, a conventional LED lighting device 100 may include a rectification unit 10, an LED light emitting unit 20, and an LED drive controller 30.
In the conventional LED lighting device 100, the rectification unit 10 generates rectified voltage Vrec through rectification of AC voltage supplied from an external power source, and outputs the rectified voltage Vrec to the LED light emitting unit 20 and the LED drive controller 30. As the rectification unit 10, any well-known rectification circuit, such as a full-wave rectification circuit or a half-wave rectification circuit, may be used. In FIG. 2, a bridge full-wave rectification circuit composed of four diodes D1, D2, D3, D4 is shown. In addition, the LED light emitting unit 20 is composed of four LED groups including first to fourth LED groups 21 to 24, which may be sequentially turned on or off under control of the LED drive controller 30. On the other hand, the conventional LED drive controller 30 is configured to control the first to fourth LED groups 21 to 24 to be sequentially turned on or off according to a voltage level of the rectified voltage Vrec.
Particularly, the conventional LED drive controller 30 increases or decreases the LED drive current according to a voltage level of input voltage (that is, rectified voltage (Vrec)) to perform constant current control in each sequential driving interval. As a result, the LED drive current has a stepped waveform approaching a sine wave, whereby power factor (PF) and total harmonic distortion (THD) of the LED lighting device can be enhanced, thereby improving power quality of the LED lighting device.
Here, operation of the conventional LED lighting device 100 will be described in more detail with reference to FIG. 3. Referring to FIG. 3, the LED drive controller 30 may include a first constant current switch SW1, a second constant current switch SW2, a third constant current switch SW3, and fourth constant current switch SW4 in order to control sequential driving of the LED groups. Specifically, in an operation interval in which the voltage level of the rectified voltage Vrec is higher than or equal to a first forward voltage level Vf1 and less than a second forward voltage level Vf2 (a first stage operation interval), the conventional LED drive controller 30 performs constant current control such that only the first LED group 21 is turned on and an LED drive current ILED becomes a first LED drive current ILED1. Similarly, in an operation interval in which the voltage level of the rectified voltage Vrec is higher than or equal to the second forward voltage level Vf2 and less than a third forward voltage level Vf3 (a second stage operation interval), the conventional LED drive controller 30 performs constant current control such that only the first LED group 21 and the second LED group 22 are turned on and the LED drive current ILED becomes a second LED drive current ILED2. Further, in an operation interval in which the voltage level of the rectified voltage Vrec is higher than or equal to the third forward voltage level Vf3 and less than a fourth forward voltage level Vf4 (a third stage operation interval), the conventional LED drive controller 30 performs constant current control such that the first to third LED groups 21 to 23 are turned on and the LED drive current ILED becomes a third LED drive current ILED3. Last, in an operation interval in which the voltage level of the rectified voltage Vrec is higher than or equal to the fourth forward voltage level Vf4 (a fourth stage operation interval), the conventional LED drive controller 30 performs constant current control such that the third constant current switch SW3 is turned off and the fourth constant current switch SW4 is turned on so as to turn on all of the first to fourth LED groups 21 to 24 and the LED drive current ILED becomes a fourth LED drive current ILED4. As shown in FIG. 3, the LED lighting device is controlled such that the LED drive current (that is, the first LED drive current ILED1) in the first stage operation interval is greater than the LED drive current (that is, the second LED drive current ILED2) in the second stage operation interval. Likewise, the LED lighting device is controlled such that the third LED drive current ILED3 is greater than the second LED drive current ILED2 and the fourth LED drive current ILED4 becomes the greatest drive current. Accordingly, the entire light output of the conventional LED lighting device 100 has a stepped waveform, as shown in FIG. 3. Accordingly, since the total number and drive current of LEDs turned on to emit light differ according to the operation intervals, the conventional LED lighting device 100 provides different light outputs according to the operation intervals, thereby causing user inconvenience due to difference in light output according to the operation intervals, and sever flicker as described above. Namely, since the conventional sequential driving type LED lighting device 100 as described above has a percent flicker of 100%, there is a need for improvement in flicker performance.
Further, the conventional LED lighting device 100 is configured to control sequential driving based on drive voltage supplied to the LED light emitting unit 20, that is, based on the voltage level of the rectified voltage Vrec. However, such a voltage detection type has a problem in that it does not satisfactorily reflect current/voltage characteristics based on temperature of LEDs. Namely, since the voltage detection type does not satisfactorily reflect I/V characteristics depending upon the temperatures of the LEDs regardless of different forward voltages of the LED groups according to “operation temperatures of LEDs”, there is a problem in that the LED drive current (LED light output) is instantaneously dropped or overshot at a time point that the operation interval is changed (for example, at a time point of changing the operation interval from the first stage operation interval to the second stage operation interval), thereby causing uneven light output of the LED lighting device 100.